1. Field of the Invention
The present invention relates to an extreme ultraviolet light source apparatus functioning as, for example, a light source for semiconductor exposure, and a method of adjusting the extreme ultraviolet light source apparatus.
2. Description of the Related Art
In recent years, as a light source for semiconductor exposure, there has been proposed an extreme ultraviolet light source apparatus (EUV light source apparatus) which emit extreme ultraviolet (EUV) light with a wavelength of 13 to 14 nm, in particular, with a wavelength of 13.5 nm. An example of the EUV light source apparatus is a discharge produced plasma (DPP) type EUV light source apparatus (see, e.g. “Present State and Future Prospect of Research of EUV (Extreme Ultraviolet) Light Source for Lithography”, Jp. Plasma Fusion Res. Vol. 79, No. 3, P219-260, March 2003).
This DPP type EUV light source apparatus comprises, for example, a container functioning as a main body, a first main discharge electrode (cathode) with an annular shape and a second main discharge electrode (anode) with an annular shape, which are disposed substantially coaxially within the container, and an EUV condensing mirror which is disposed under the first and second main discharge electrodes. In a method of emitting EUV light, a large pulsating current is caused to flow between the first and second main discharge electrodes, for example, in the state in which an EUV radiation seed is introduced in the container. Thereby, the EUV radiation seed, which is introduced in the vicinity of the first and second main discharge electrodes, is heated and excited, and a high-density high-temperature plasma is generated in the vicinity of the first and second main discharge electrodes. EUV light, which is radiated from the high-density high-temperature plasma, is reflected by the EUV condensing mirror and is emitted to an exposing device.
However, if the high-density high-temperature plasma is generated, such a problem arises that debris, for instance, metal powder, which is produced by sputtering by the high-density high-temperature plasma, or debris occurring from the radiation seed, may damage the EUV condensing mirror. In order to prevent damage to the EUV condensing mirror, a debris trap for passing only EUV light is formed between the first and second main discharge electrodes and the EUV condensing mirror. As the debris trap, for example, a structure called “foil trap” has been proposed (see, e.g. Jpn. PCT National Publication No. 2004-214656).
On the other hand, in some cases, the high-density high-temperature plasma, which is the light source of EUV light, may melt and deform the nearby first and second main discharge electrodes, and may vary the position of electric field. As a result, the position of generation of the high-density high-temperature plasma shifts and the position of the luminescent point of EUV light that is emitted from the plasma may deviate from the central position of the foil trap. Hence, such a problem occurs that the amount of light which can pass through the foil trap decreases.
Thus, in the conventional EUV light source apparatus, it is difficult to obtain EUV light of a stable and desired light amount.